Fix T58550 Dragged in images dont overlap properly

This patch adds a new "Use Alpha" option on image empties to avoid ordering
issue of reference images.

If ordering is not important, "Use Alpha" can be enabled to provide
transparency and alpha blending support.

2 files changed, 30 insertions, 14 deletions

diff --git a/source/blender/draw/modes/object_mode.c b/source/blender/draw/modes/object_mode.c
index 4fad882a7b5..5cacfd0e941 100644
--- a/source/blender/draw/modes/object_mode.c
+++ b/source/blender/draw/modes/object_mode.c
@@ -927,6 +927,7 @@ static void DRW_shgroup_empty_image(
 	/* Calling 'BKE_image_get_size' may free the texture. Get the size from 'tex' instead, see: T59347 */
 	int size[2] = {0};
 
+	const bool use_alpha_blend = (ob->empty_image_flag & OB_EMPTY_IMAGE_USE_ALPHA_BLEND) != 0;
 	GPUTexture *tex = NULL;
 
 	if (ob->data != NULL) {
@@ -943,38 +944,40 @@ static void DRW_shgroup_empty_image(
 	float image_aspect[2];
 	image_calc_aspect(ob->data, size, image_aspect);
 
-	/* OPTI(fclem) We need sorting only for transparent images. If an image as no alpha channel and
-	 * ob->col[3] == 1.0f,  we could remove it from the sorting pass. */
-
-	if (tex && (ob->color[3] > 0.0f) && BKE_object_empty_image_data_is_visible_in_view3d(ob, rv3d)) {
-		DRWShadingGroup *grp = DRW_shgroup_create(sh_data->object_empty_image, sgl->image_empties);
-		DRW_shgroup_uniform_texture(grp, "image", tex);
+	{
+		DRWShadingGroup *grp = DRW_shgroup_create(sh_data->object_empty_image_wire, sgl->non_meshes);
 		/* TODO(fclem) implement DRW_shgroup_uniform_vec2_copy */
 		DRW_shgroup_uniform_float_copy(grp, "aspectX", image_aspect[0]);
 		DRW_shgroup_uniform_float_copy(grp, "aspectY", image_aspect[1]);
 		DRW_shgroup_uniform_int_copy(grp, "depthMode", ob->empty_image_depth);
 		DRW_shgroup_uniform_float(grp, "size", &ob->empty_drawsize, 1);
 		DRW_shgroup_uniform_vec2(grp, "offset", ob->ima_ofs, 1);
-		DRW_shgroup_uniform_vec4(grp, "objectColor", ob->color, 1);
+		DRW_shgroup_uniform_vec3(grp, "color", color, 1);
 		if (sh_cfg == GPU_SHADER_CFG_CLIPPED) {
 			DRW_shgroup_world_clip_planes_from_rv3d(grp, DRW_context_state_get()->rv3d);
 		}
-		DRW_shgroup_call_add(grp, DRW_cache_image_plane_get(), ob->obmat);
+		DRW_shgroup_call_add(grp, DRW_cache_image_plane_wire_get(), ob->obmat);
 	}
 
-	{
-		DRWShadingGroup *grp = DRW_shgroup_create(sh_data->object_empty_image_wire, sgl->non_meshes);
-		/* TODO(fclem) implement DRW_shgroup_uniform_vec2_copy */
+	if (!BKE_object_empty_image_data_is_visible_in_view3d(ob, rv3d)) {
+		return;
+	}
+
+	if (tex && ((ob->color[3] > 0.0f) || !use_alpha_blend)) {
+		DRWShadingGroup *grp = DRW_shgroup_create(sh_data->object_empty_image,
+		                                          (use_alpha_blend) ? sgl->image_empties : sgl->non_meshes);
 		DRW_shgroup_uniform_float_copy(grp, "aspectX", image_aspect[0]);
 		DRW_shgroup_uniform_float_copy(grp, "aspectY", image_aspect[1]);
 		DRW_shgroup_uniform_int_copy(grp, "depthMode", ob->empty_image_depth);
 		DRW_shgroup_uniform_float(grp, "size", &ob->empty_drawsize, 1);
 		DRW_shgroup_uniform_vec2(grp, "offset", ob->ima_ofs, 1);
-		DRW_shgroup_uniform_vec3(grp, "color", color, 1);
+		DRW_shgroup_uniform_texture(grp, "image", tex);
+		DRW_shgroup_uniform_vec4(grp, "objectColor", ob->color, 1);
+		DRW_shgroup_uniform_bool_copy(grp, "useAlphaTest", !use_alpha_blend);
 		if (sh_cfg == GPU_SHADER_CFG_CLIPPED) {
 			DRW_shgroup_world_clip_planes_from_rv3d(grp, DRW_context_state_get()->rv3d);
 		}
-		DRW_shgroup_call_add(grp, DRW_cache_image_plane_wire_get(), ob->obmat);
+		DRW_shgroup_call_add(grp, DRW_cache_image_plane_get(), ob->obmat);
 	}
 }
 
diff --git a/source/blender/draw/modes/shaders/object_empty_image_frag.glsl b/source/blender/draw/modes/shaders/object_empty_image_frag.glsl
index e47b28d80c6..c20d70e3b06 100644
--- a/source/blender/draw/modes/shaders/object_empty_image_frag.glsl
+++ b/source/blender/draw/modes/shaders/object_empty_image_frag.glsl
@@ -12,13 +12,26 @@ uniform sampler2D image;
 #endif
 
 uniform int depthMode;
+uniform bool useAlphaTest;
 
 void main()
 {
 #ifdef USE_WIRE
 	fragColor = finalColor;
 #else
-	fragColor = finalColor * texture(image, texCoord_interp);
+	vec4 tex_col = texture(image, texCoord_interp);
+	fragColor = finalColor * tex_col;
+
+	if (useAlphaTest) {
+		/* Arbitrary discard anything below 5% opacity.
+		 * Note that this could be exposed to the User. */
+		if (tex_col.a < 0.05) {
+			discard;
+		}
+		else {
+			fragColor.a = 1.0;
+		}
+	}
 #endif
 
 	if (depthMode == DEPTH_BACK) {